Pipe used in the oil and gas industry can be about 2 feet in diameter and up to about 30 feet in length. These pipes typically have threading cut on an inner or outer diameter of each end for use in connecting adjacent sections of pipe together. Such large pipes, however, present challenges in cutting the threads in an efficient, safe, and cost effective manner. Current equipment for machining pipe threads on these large pipes is either a commercially available lathe or a horizontal machining center. However, each machine has constraints that render the machining of large size work pieces, such as pipeline for the oil and gas industry, difficult and time consuming.
One approach of threading large size pipe is to use the commercially available lathe. This equipment rotates the pipe along a horizontal axis thereof, and a stationary tool cuts the thread on the end of the pipe as it is rotated. In order to secure the pipe to the lathe, the one end of the pipe that is rotated is passed through the center of the lathe headstock and is secured thereto by clamping using chucks with gripping jaws located on the front and rear of the headstock. In order for the lathe to function correctly, the pipe must be centered on the lathe horizontal turning axis. The centering of the pipe along the lathe axis is a time consuming, labor intensive task that must be completed each time a new pipe is installed into the lathe. The centering operation is even more difficult with the large size pipes used in the oil and gas industry.
In order to center pipe on the lathe, current methods use a fixed indicator placed in the end of the pipe at the headstock. The pipe is then slowly rotated by hand in the chuck until the inner surface of the pipe contacts the indicator. When the indicator signals that the pipe is not rotating on the center of the lathe axis, the chuck jaws must then be loosened and the pipe readjusted. This trial and error process is then repeated until the pipe rotates on the center of the lathe axis as signaled by the indicator. This centering process is an extremely time consuming task that must be performed each time before the machining can begin on a new pipe. Only one end of the pipe is threaded so that to thread the other end of the pipe, it must be turned through 180 degrees to position the pipe end previously in the support area at the chuck.
Even after the large diameter pipe has been centered so that proper lathe cutting can be performed, rotating such large size pipe (i.e., about 30 feet long and about 2 feet in diameter) can also create safety concerns as well. For instance, sufficient guarding must be provided around the lathe and work piece in order to protect the operators and surrounding areas from the rotating part during machining. In addition, many pipes may be unbalanced, which creates technical and other safety issues when rotating. Rotating an unbalanced pipe at high speeds can put stress on the headstock bearings causing premature failure. The unbalanced pipe can also cause accuracy issues by causing an out-of-round condition on the threading machining. In response to unbalanced work pieces, operators need to slow the rotational speed of the pipe in the headstock in order to minimize any effect of the out-of-round condition. A large pipe also has considerable inertia to be overcome to start and stop of its rotation. Slower lathe speeds result in less than optimal cutting conditions, and also reduce tool life and add cycle time to the overall process.
Another conventional method of threading pipe is to use the horizontal machining center. In this method, the pipe is secured to a table so as not to rotate, and then moved horizontally relative to the cutting tool. The horizontal machining center provides advantages over the lathe system because the pipe does not rotate. However, current horizontal machine centers also have shortcomings such as size of the movable worktables when machining large size work pieces, such as the 30 foot long pipe for the oil and gas industry.
In a typical horizontal machining center, the machine table is an axis that must be able to move longitudinally as part of the machining operation. That is, the table must move horizontally along a feed axis so as to feed the part into the cutting head. Current movable tables are restricted in size, and only limited sizes of pipe can be mounted thereon. Existing tables are configured to accept part lengths up to about 10 feet. Constructing larger moveable tables, such as tables capable of handing a 30 foot long pipe, is not a cost effective solution.
In addition to physical constraints with the size of work piece suitable for cutting on the horizontal machining center, the thread cut by the horizontal machining center is less preferred. The threading of the inner or outer diameter of the pipe on current horizontal machining centers is through a circular interpolation movement of the tool about the non-rotating pipe rather than a true circular movement as obtained with the lathe. The circular interpolation is created by moving the machine head along two linear axes in small step movements around the circumference of the pipe to position the cutting tool at the depth or in-feed for cutting the thread. Since the thread is generated using a combination of these small linear steps, the quality of the thread and accuracy is not as good as a thread generated on a rotational axis such as used on a lathe.
When cutting a thread on piping, the horizontal machining center also tends to be more expensive to operate. For example, because of the circular interpolation movement, the cycle or cutting time is much longer compared to turning the thread on a lathe because the metal removal rate is less with thread milling (i.e., horizontal machining center) than with turning on a lathe. The perishable thread mill tool used on the machining center is also more expensive than indexable inserts used on threading tool holders in a lathe. The thread mill tools are specially ground to the thread form that they are needed to generate and must be reground when worn or discarded completely if broken. The indexable inserts commonly used in lathe operations, on the other hand, are readily available in the industry for many types of thread forms and easily changed when they are worn or broken.
The drawbacks of circular interpolation may be overcome by the use of a rotary spindle head configured to rotate a cutting tool about a rotational axis to cut a circular thread. An example of such a head is the U-TRONIC head available from D'Andrea, S.P.A. The cutting tool is positioned radially along a feed-out axis to engage the work piece at the proper thread depth and then rotated to cut the thread. However, in such systems the pipe is translated along its longitudinal axis during machining, creating the drawbacks discussed above associated with moving the machine table during the machining operation.
The horizontal machine centering also suffers from similar trial and error shortcomings described above in connection with the lathe when centering of the work piece. Current methods require a similar indicator that is mounted to the headstock center line that is swept around the inner diameter of the pipe by hand. If the indicator signals that the pipe is not on-center with the headstock then the pipe must be adjusted. As with the lathe, this process is repeated until a sweep of the indicator signals that the pipe inner walls are on the center of the machine axis. This manual process is time consuming and tedious and also must be completed each time a new pipe is to be cut.
Therefore, there is a desire for a machining system and a method of use therefor, that overcomes many of the disadvantages of these prior art lathe and horizontal machining centers heretofore used with large size work pieces, such as 30 foot long pipe designed for the oil and gas industry.